Socio-economic pressures and customer needs render night work imperative in many occupations. The goals of "Healthy People 2000" include--to improve occupational safety and health by reducing work-related injuries and deaths. Sleep deprivation, common in night workers, is a major cause of accidents and fatalities. Alertness and sleep are normally rhythmic events synchronized to the light and dark phases of the 24 hour solar cycle. Rotating or permanent night work cause dissociation of rhythms from the 24 hour light/dark phases with resultant poor sleep, diminished alertness, and mood disturbances. Melatonin is effective in accelerating adjustment to alterations in the 24 hour light/dark cycle in certain conditions, e.g., jet lag and blindness. The proposed project will test the hypothesis that melatonin accelerates the adjustment of workers to a reversal in the activity/sleep cycle from daytime to night work by synchronizing sleep to the desired schedule and consequently improving alertness and mood during the waking hours. The hypothesis will be tested in an actual work place, a hospital. Subjects will be physicians in training whose work requires intense alertness and vigilance. The design includes two treatment phases for each subject, melatonin and placebo, and two respective baseline phases, one for each treatment; each phase lasts two weeks. Melatonin or placebo will be administered daily in the morning. Outcome measures include: (1) sleep characteristics obtained by diary and wrist actigraphy, (2) alertness/vigilance assessed by the Conner's continuous performance test, and (3) mood assessed by the Profile of Mood States. The proposed model of shift work and the outcome measures are not job-specific, hence, the results of the study can be generalized to other occupations which require high levels of vigilance and alertness. The expected benefits of the proposed intervention may lead to development of new strategies for adjustment to night work, resulting in increased safety and reduced accidents and fatalities related to sleep deprivation in night workers.